The 'platform of the Alt Right' isn't happy with him.

Donald Trump has suffered “the first major crack” in his coalition of support, after a usually loyal website ran a caustic headline attacking him for apparently ditching a campaign promise.

Breitbart News, the Alt Right website catapulted into the mainstream by Trump’s ascension, ran a headline “BROKEN PROMISE” after Trump’s spokeswoman said he would not pursue the Hillary Clinton prosecution he vowed to throughout the campaign.

The FBI concluded Clinton should face no action over her alleged handling of classified information via emails.

On Tuesday, Trump spokeswoman Kellyanne Conway told MSNBC Trump’s “decision” to not pursue a prosecution could help Clinton “heal”.

“I think Hillary Clinton still has to face the fact that the majority of Americans don’t find her to be honest and trustworthy,” she said. “But if Donald Trump can help her heal, then perhaps that is a good thing.”

The comments section on Breitbart’s short article on it, Trump supporters lost it, with one crying: “This is total BULLSHlT. Trump keep your fuqing [sic] word.”

Daily Beast reporter Olivia Messer tweeted the moment from The Lion King, when Simba feels the spirit of his dead father Mufasa has abandoned him, to explain the psychological link between Breitbart and Trump’s campaign promises.

Breitbart, upon finding out Trump isn't actually going to assign a special prosecutor to Hillary's email case: pic.twitter.com/MmtK9bvEvJ